Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Algal overgrowth of alcyonacean soft corals

Colonies of the soft coral Lobophytum pauciflorum (Ehrenberg, 1834) (Coelenterata: Octocorallia: Alcyonacea: Alcyoniidae), some of which were heavily overgrown by the algae Ceranium flaccidum (Kuetzing) Ardissone and Enteromorpha sp., and other minor epizoites, were collected at Taylor Reef (17°50S; 146°35E) in the Great Barrier Reef. Overgrown colonies contained the diterpene 2-epi-sarcophytoxide as the major secondary metabolite, while conspecific colonies with clean polyparies contained two diterpenes in approximately equal amounts: 14-hydroxycembra-1,3,7,11-tetraene and 15-hydroxycembra-1,3,7,11-tetraene. By contrast, twenty conspecific pairs of overgrown and clean colonies of other alcyoniid soft corals collected from Pelorus Channel, Palm Island Group (18°34S; 146°29E), showed no chemical differences within in the pairs. Cultures of a common species of Ceramium [C. codii (Richards) Mazoyer] were incubated with different concentrations of nine soft-coral-derived diterpenes and significant algal growth inhibition was observed in many cases. It appears that terpenoids from soft coral may contribute to the lack of epizoic organisms on soft-coral polyparies.     

Laboratory observations on spawning,fecundity and early development of a mesopelagic ostracod,Conchoecia pseudodiscophora,from the Japan Sea

The fate of the polycyclic aromatic hydrocarbon [³H]benzo[a]pyrene (BaP) was examined in two species of scleractinian corals, Favia fragum (Esper) and Montastrea annularis (Ellis and Solander), which were collected in the patch reefs surrounding Alina's Reef (25°23.25N; 80°09.8W) in Biscayne National Park, Florida, USA, in July, 1990. Corals were exposed to initial concentrations of 5 g/l in a simple static system for 25 h. BaP uptake was estimated from the disappearance of BaP from the water. Uptake rates were 6.5±0.7 and 10.8±0.2 g BaP cm^-2h^-1 for F. fragum and M. annularis, respectively, at initial BaP concentrations and were directly proportional to the concentration of BaP in the water. The separation of zooxanthellae from coral tissue revealed that zooxanthellae can accumulate up to 53 and 64% of the total BaP-derived radioactivity present in F. fragum and M. annularis, respectively. Both corals metabolized BaP slowly, as most of the accumulated radioactivity was present as the unmetabolized chemical. However, aqueous and organic-soluble metabolites were found in both the animal and zooxanthellae fractions. Analysis by high-performance liquid chromatography (HPLC) revealed that both species of corals metabolized BaP to various tetrols, triols, dihydrodiols, quinones and phenols, although the pattern of metabolites differed between species. Zooxanthellae contained some of the same Phase I metabolites found in the animal tissue; however, tetrols and triols were absent in extracts from the zooxanthellae. The elimination of BAP from corals was also slow; approximately 38 and 65% of the accumulated radioactivity was still present in F. fragum and M. annularis, respectively, 144 h following the transfer of exposed corals to an uncontaminated flow-through seawater system.     

Light harvesting by wavelength transformation in a symbiotic coral of the Red Sea twilight zone

We report an extraordinary depth range for Leptoseris fragilis (Milne Edwards and Haime), a reef building coral of the Red Sea living in cytosymbiosis with zooxanthellae. The coral harbours an as yet unknown pigment system. We suggest that the heterotrophic host — the coral — provides its photoautotrophic symbionts with additional light. The supplementary light is provided by host pigments which transform light of short wavelengths into suitable wavelengths for photosynthesis, thus amplifying and increasing the transfer of photoassimilates from the zooxanthellae to the host.     

Transformation of soft coral (Coelenterata: Octocorallia) terpenes by Ovula ovum (Mollusca: Prosobranchia)

The faecal pellets from specimens of the prosobranch mollusc Ovula ovum found feeding on the soft coral Sarcophyton sp. at Eclipse Island, Palm Island Group (18°46S; 146°33E) in November 1980 were analysed. The only terpene present in the faeces, 7,8-deoxysarcophytoxide, differed from the major constituent of the soft coral, sarcophytoxide, suggesting that the latter had been transformed into the former within the cowrie. This transformation is not trivial, and could not be produced simply by acid catalysis. Subsequent analysis of tissues dissected from different regions of O. ovum indicates that the transformation is probably effected by enzymes in the digestive diverticula stomach region of the prosobranch. The transformed compound is significantly less toxic to the mosquito fish Gambusia affinis Baird and Girard than the ingested compound.     

Effects of ambient levels of solar ultraviolet radiation on zooxanthellae and photosynthesis of the reef coral Montipora verrucosa

Paired flat plates of the hermatypic coral Montipora verrucosa from Kaneohe Bay, Oahu, Hawaii, were acclimated to photosynthetically active radiation (PAR) only and to full sunlight (PAR+UV) for several weeks in the summer of 1990. After the acclimation period, photosynthesis, both in PAR-only and PAR+UV as well as dark respiration were measured. Levels of the UV-absorbing compounds, S320, density of zooxanthellae, and chlorophyll a concentration were determined. Corals acclimated in PAR+UV had higher levels of the UV-protective compounds and lower areal zooxanthellae densities than corals acclimated in PAR-only. Chlorophyll a per unit volume of coral host and per algal cell did not differ between corals from the two acclimation treatments. Corals acclimated to PAR+UV displayed higher photosynthesis in full sunlight than corals acclimated to PAR-only, but when photosynthesis was measured in the light regime to which the corals had been acclimated, there were no differences in photosynthesis. Dark respiration was the same for corals from the two acclimation treatments regardless of the light quality immediately preceding the dark period.Contribution No. 902 HIMB     

What is the sex ratio of harpacticoid copepods in the deep sea?

The sex ratios of deep-sea harpacticoids have been thought to be greatly skewed toward females. The representation of males is notably more equitable (approximately 1:2) at three deep-sea sites (San Diego Trough: 32°52.4N, 117°45.5W, 1 050 m, January 1987; Porcupine Seabight: 51°36.85N, 12°57.30W, 1 369 m, August 1984; northwest Atlantic, 40°27N, 62°20W, 4 820 m, July 1982, June 1983). This ratio conceals extensive variation at the species level. For some species, males are unknown. For other species, males are more numerous than females. Sampling bias against males can occur and may explain the more extreme reports of male rareness in the literature.     

Effect of freezing on photosynthesis of intertidal macroalgae: relative tolerance of Chondrus crispus and Mastocarpus stellatus (Rhodophyta)

The effect of freezing on photosynthetic metabolism was studied in the red algae, Chondrus crispus and Mastocarpus stellatus. Plants of both species were collected from the intertidal at Chamberlain or Kresge Point, Maine, USA (43°56N, 69°54W) between February and March 1987. Photosynthetic rates were measured immediately after freezing at-20°C and following recovery periods in seawater. Photosynthesis in C. crispus declined rapidly following freezing, falling to 70% of control values within 1 h and 30% after 3 h exposure. Minimum photosynthetic rates (7 to 9% of controls) occurred following freezing exposures of 12 h or more. Full photosynthetic recovery in C. crispus after 3 h at-20°C required 48 h. Photosynthesis in C. crispus did not fully recover in plants frozen for 6 h or more. In contrast, photosynthesis in M. stellatus was relatively unaffected by freezing exposures of <12 h. Twelve hours or more at-20°C reduced photosynthesis to 55% of controls. Photosynthesis in M. stellatus fully recovered from 24 h at-20°C within 24 h. In both species the reduction of photosynthesis by freezing was associated with damage to the plasma membrane and reduced efficiency of energy transfer from phycobilisomes to chlorophyll a, but did not appear to involve ribulose-1,5-bisphosphate carboxylase oxygenase activity. The freezing tolerance of C. crispus and M. stellatus positively correlates with their respective intertidal distributions, suggesting that freezing may be involved in controlling the distributions of these species on the shore.     

Food and daily ration of the rock soleLepidopsetta bilineata (Pleuronectidae) in the eastern Bering Sea

A total of 129 specimens ofLepidopsetta bilineata (Ayres) were collected in the eastern Bering Sea (57°02–58°11N; 161°58–164°38W) between 28 August and 7 September 1984. Examination of its stomach and intestine contents revealed that polychaetes, gammarid amphipods and echiuroids constitute the bulk of its diet. Prey type is related to fish size. Juveniles consume more mobile prey, such as small crustaceans (gammarid amphipods, cumaceans, carideans), while adults, although still continuing to feed on gammarid amphipods, preferentially feed on infaunal organisms such as polychaetes, echiuroids and mollusks. Some prey, such as echinoderms, fishes and urochordates, were encountered only in stomachs, which resulted in a higher index of feeding diversity for this organ. Rock sole feeds primarily during daylight hours, peaking at dusk. Its daily ration was estimated as 0.49, 1.12, and 1.61% of fish body weight from contents of stomach, intestine, and stomach plus intestine, respectively.     

Movement in soft corals: An interaction between Nephthea brassica (Coelenterata: Octocorallia) and Acropora hyacinthus (Coelenterata: Scleractinia)

An interaction between the alcyonarian octocoral Nephthea brassica Kukenthal, 1904 and the scleractinian Acropora hyacinthus Dana was photographed and the assemblage collected at Rib Reef (18°15S; 146°45E), Central Region of the Great Barrier Reef, in September 1980. The interaction, which did not involve allelopathy, was characterised by extensive deposition of undifferentiated aragonite by the scleractinian in the vicinity of the soft coral, which moved across the surface of the acroporid. The basal attachment region of the nephtheid showed structural features which permitted movement. The net effect of this interaction was a series of divided trails across the surface of the acroporid plate evidencing the movement through growth and asexual division of the parent colony. These trails were susceptible to secondary colonisation by epifauna and algae which may reduce the fitness of the acroporid. This movement appears to afford N. brassica a distinct advantage in competing for space with acroporid corals.     

Effects of feeding frequency and symbiosis with zooxanthellae on nitrogen metabolism and respiration of the coral Astrangia danae

Colonies of the temperate coral Astrangia danae occur naturally with and without zooxanthellae. Basal nitrogen excretion rates of nonsymbiotic colonies increased with increasing feeding frequency [average excretion rate was 635 ng-at N (mg-at tissue-N)^-1 h^-1]. Reduced excretion rates of symbiotic colonies were attributed to N uptake by the zooxanthellae. Nitrogen uptake rates of the zooxanthellae averaged 8 ng-at N (10⁶ cells)^-1 h^-1 in the dark and 21 ng-at N (10⁶ cells)^-1 h^-1 at 200 Ein m^-2 s^-1. At these rates the zooxanthellae could provide 54% of the daily basal N requirement of the coral if all of the recycled N was translocated. Basal respiration rates were 172 nmol O₂ cm^-2 h^-1 for starved colonies and 447 nmol O₂ cm^-2 h^-1 for colonies fed three times per week. There were no significant differences between respiration rates of symbiotic and nonsymbiotic colonies. N excretion and respiration rates of fed (symbiotic and nonsymbiotic) colonies increased greatly soon after feeding. N absorption efficiencies decreased with increasing feeding frequency. A N mass balance, constructed for hypothetical situations of nonsymbiotic and symbiotic (3×10⁶ zooxanthellae cm^-2) colonies, starved and fed 15 g-at N cm^-2wk^-1, showed that the presence of symbionts could double the N growth rate of feeding colonies, and reduce the turnover-time of starved ones, but could not provide all of the N requirements of starved colonies. Rates of secondary production, estimated from rates of photosynthesis and respiration were similar to those estimated for reef corals.     

A brominated secondary metabolite synthesized by the cyanobacterial symbiont of a marine sponge and accumulation of the crystalline metabolite in the sponge tissue

The dictyoceratid marine sponge Dysidea herbacea (Keller, 1889) is common in shallow waters of the tropical Pacific Ocean. Polybrominated biphenyl ethers such as 2-(2,4-dibromophenyl)-4,6-dibromophenol (1) are characteristic secondary metabolites of some specimens of this sponge and may represent as much as 12% of the dry weight. We have found 1 to be deposited as conspicuous crystals throughout the sponge tissue. The dominant prokaryotic endosymbiont in the mesohyl of the sponge is a filamentous cyanobacterium (Oscillatoria spongeliae), although a vacuole-containing, heterotrophic bacterium is also present. The cyanobacteria were separated from the sponge cells and heterotrophic bacteria by flow cytometry. Coupled gas chromatography—mass spectrometry and proton nuclear magnetic-resonance spectroscopy revealed that the major brominated Compound 1 isolated from the intact symbiotic association is found in the cyanobacteria and not in the sponge cells or heterotrophic bacteria. This suggests that the production of the compound is due to the cyanobacterium, and not to the sponge or symbiotic heterotrophic bacteria, as had been suggested earlier.     

Ammonium enhancement of dark carbon fixation and nitrogen limitation in zooxanthellae symbiotic with the reef coralsMadracis mirabilis andMontastrea annularis

The nutrient status (limitation vs sufficiency) of dinoflagellates (zooxanthellae) symbiotic with reef corals in Bermuda was assessed in 1989 and 1990 by measuring the enhancement of dark carbon fixation with 20 M ammonium by isolated symbionts. A colony ofMadracis mirabilis was kept in the laboratory and fed daily or starved for one month. Symbionts from fed portions of the colony had ammonium-enhancement ratios (NH _4dark ⁺ ; SW_dark;SW=seawater without added ammonium) similar to those of the original field population (1.2 to 1.3). Ammonium-enhancement ratios increased with starvation of the host (x1.7) as did values forV _D:V _L [(ammonium dark rate-seawater dark rate): light rate in seawater]. Both parameters indicated decreasing nitrogen sufficiency of the algae when the host was not fed, but starvation appeared to affect these algae less than symbionts of sea anemones. Field samples of zooxanthellae fromM. mirabilis (Three Hill Shoals and Bailey's Bay Flats) yielded results similar to those for fed corals, but those taken from Bailey's Bay Flats in May 1990 yielded exceptionally high values for enhancement (>3) andV _D:V _L indicating pronounced nitrogen limitation at the time of sampling. We sampled zooxanthellae from populations ofMontastrea annularis at 8 m (Three Hill Shoals) and 24 m (Soldier's Point) depths. Enhancement andV _D:V _L values for zooxanthellae from the 8 m corals were density-dependent: symbionts from corals with normal symbiont densities displayed the most nitrogen limitation (enhancement values=1.4 to 2.0), while those from bleached corals with lower density exhibited enhancement andV _D:V _L values typical of nitrogen-sufficient algae. Symbionts isolated from the 25 m corals yielded the highest values, and appeared to exhibit the least nitrogen-sufficiency for this species.     

Daily photosynthesis,respiration, and carbon budgets in a tropical marine jellyfish (Mastigias sp.)

From measured diel photosynthesis and respiration rates, using oxygen electrodes, estimates of carbon flux between symbiotic algae (zooxanthellae) and host animal are presented for the marine scyphomedusan Mastigias sp. from a marine lake in Palau, Western Caroline Islands, during February and March 1982. The carbon budgets calculated for these lake medusae indicate that carbon fixed photosynthetically by zooxanthellae and made available to the host may satisfy up to 100% of the host's daily metabolic carbon demand (CZAR). The stable carbon isotope (¹³C) signature of the mesogleal carbon of lake Mastigias sp. was close to that of the zooxanthellae, supporting the interpretation that while these medusae may feed holozoically, some of their carbon comes from their symbionts. The diel photosynthesis, respiration, and preliminary estimates of carbon budgets of three individuals of another ecotype of Mastigias sp. collected from nearby oceanic lagoons are also given. Photosynthesis of lagoon medusae was generally greater than that for lake medusae of similar size, and lagoon medusae were phototrophic with respect to carbon, with commensurately greater CZAR values. Carbon translocated from the symbiotic algae also may contribute to the growth requirements of both lake and lagoon medusae. From carbon flux data, the lake jellyfish were estimated to contribute about 16% to the total primary productivity of their marine lake habitat.     

Light responses of a scleractinian coral (Plerogyra sinuosa)

During daytime Plerogyra sinuosa Dana displays globular expandable tentacles (bubbles) which foster the photosynthetic ability of the coral. Adaptational responses of this coral to different depths (5–25 m) and light conditions were investigated by photosynthetic pigment analysis, insitu measurements of oxygen production, transplantation and shading experiments. Pigment concentrations per unit tissue dry weight were variable, but unrelated to depth. Pigment concentrations per zooxanthellae cell remained constant and bubble size increased with depth. Light intensity at 25 m was 20 to 25% of the 5-m value, but daily integrated rates of photosynthesis were 65% of the 5-m rates, indicating a higher light utilization efficiency in deeper corals. Coral heads transplanted from 25 to 5 m died within 20 d if not protected against UV-radiation, but corals transplanted from 5 to 25 m acclimatized to the new light condition. Photosynthetic oxygen production and bubble size increased in shaded, sun-adapted corals within 60 min and decreased in sun-exposed, shade-adapted corals. The variable bubble size is interpreted as an adaptational mechanism to optimize light exposure of zooxanthellae.     

Sea-water temperature and spawning behaviour in the seastar Marthasterias glacialis

The seastar Marthasterias glacialis (L.) was seen to spawn in two sea inlets, Lough Hyne (9°18W; 51°30N) and Mulroy Bay (7°42W; 55°12N), Ireland. Spawning occurred during the afternoon and early evening between 13.30 and 21.30 hrs British Summer Time (BST) on sunny days during July and August, 1978. Groups remained assembled for days prior to spawning. Solitary and grouped seastars arched themselves and released their gametes freely. Local moderate onshore winds on sunny days resulted in increases of sea temperature which often induced spawning. Males observed spawning ranged from 2.5 to 26 cm and females from 9 to 22 cm in diameter over the period 1978–1985.     

Relationship of daily and circatidal activity rhythms of the fiddler crab,<Emphasis Type="Italic"> Uca princeps</Emphasis>, to the harmonic structure of semidiurnal and mixed tides

Intertidal organisms may employ circatidal rhythms to track the tidal cycle, but tidal patterns may vary within a species range and necessitate adaptation to the local tides. Circatidal rhythms were examined in populations of the eastern Pacific fiddler crab Uca princeps (Smith) from four sites with differing tidal characteristics, La Paz (24°10N; 110°21W), San Blas (21°33N; 105°18W) and Manzanillo (19°6N; 104°24W), Mexico (lower amplitude, mixed semidiurnal tides) and Mata de Limon, Costa Rica (9°55N; 84°43W) (high-amplitude, semidiurnal tides). Local tides were characterized by harmonic constants of M₂, S₂, K₁, and O₁, partial tides that largely determine their semidiurnal and diurnal features. Rhythmic structure in continuously recorded locomotor activity of individual crabs held under laboratory conditions was described by cosinor and periodogram methods of time-series analysis. Both daily and circatidal rhythms were found in crabs studied in light–dark cycles set to local conditions at the time of collection. Crabs at all four sites shared a tendency toward bimodality, with a mid-morning activity peak and varying degrees of nocturnal activity. Circatidal rhythms closely matching the period of the 12.42-h M₂ partial tide were consistently present at all sites except Manzanillo. At Mata de Limon, the circatidal rhythm clearly dominated locomotor activity, but was strongly modulated by a daily rhythm in a repeating pattern at a semilunar interval. In contrast, the amplitude of the daily rhythm was higher than that of the circatidal rhythm in crabs from the three mixed tide sites on the Mexican coast, where the tidal pattern is dominated by a diurnal inequality arising from the diurnal K₁ and O₁ partial tides. These results suggest that populations of U. princeps use both daily and circatidal timing systems to track local forms of the tide generated by their M₂, S₂, K₁, and O₁ geophysical counterparts.Communicated by J.P. Grassle, New Brunswick     

Evidence for facultative heterotrophy in cultured zooxanthellae

The locus of symbiotic dinoflagellates within host cells provides a habitat which could potentially be exploited by the alga through heterotrophic uptake of host-derived organic substrates. Using zooxanthellae (Symbiodinium sp.) isolated from the tropical sea anemone Aiptasia pulchella collected from Kaneohe Bay, Hawaii, the effect of various potential organic substrates on growth in vitro was assessed in Erdschreiber seawater medium (ES) supplemented with organic compounds. Zooxanthellae maintained at 5 to 7 E m^-2 s^-1 (below compensation irradiance) grew heterotrophically when supplied with 100 M glycerol, glycolate, acetate, malate, or propionate, and grew in darkness on 100 M propionate. Zooxanthellae exposed to irradiance below compensation were able to utilize carbon sources in the unsupplemented ES medium for slow growth, but generally the growth rate of cultured zooxanthellae was a function of incubation irradiance. Zooxanthellae incubated for 10 wk in unsupplemented ES at 5 to 7 E m^-2 s^-1 were capable of growth at this low irradiance, but were also capable of net photosynthetic oxygen production at higher irradiances. This suggests that zooxanthellae can be photoautotrophic or facultatively heterotrophic. An estimate for the duration of mitosis (t _d ) is made on the basis of growth rate of cultured zooxanthellae in log-phase; this estimate of t _d =4.88 h is less than half the estimated t _d for zooxanthellae in situ.     

Zooxanthellae providing assimilatory power for the incorporation of exogenous acetate in Heteroxenia fuscescens (Cnidaria: Alcyonaria)

The soft coral Heteroxenia fuscescens (Ehrb.) and its isolated zooxanthellae (endosymbiotic dinoflagellates) were investigated with particular regard to uptake and utilization of exogenously supplied ¹⁴C-acetate in the light and in the dark. The incorporation of ¹⁴C from ¹⁴C-acetate into the host tissue and into the zooxanthellae was consistently much higher in the light than in the dark. The incorporated ¹⁴C-acetate was rapidly metabolized by the host and algae and was recovered from different assimilate fractions. The major proportion of radiocarbon from metabolized ¹⁴C-acetate was located in host tissue. The CHCl₃-soluble fraction composed of diverse lipids showed the strongest ¹⁴C-labelling. Zooxanthellae isolated prior to incubation accounted for about 80% of the acetate incorporation recorded for zooxanthellae in situ (in vivo). It is concluded from a comparison of acetate incorporation and conversion under light and dark conditions that most of the lipid reserve of the host tissue originates from fatty acids, which are synthesized within the algal symbionts and are then translocated to the heterotrophic partner via extrusion. The acetate units needed for lipid synthesis are obtained by absorption of free acetate from dissolved organic matter (DOM) in the seawater as well as by photosynthetic assimilation of inorganic carbon. Thus, in H. fuscescens, lipogenesis is operated as a light-driven process to which the zooxanthellae considerably contribute assimilatory power by performing fatty acid synthesis and translocation of lipid compounds to their intracellular environment (host cell). A metabolic scheme is proposed to account for the different pathways of carbon conversion observed in H. fuscescens. The incubations took place in August 1980 and the analytical part from October 1980 to January 1984.     

A “CO2 supply” mechanism in zooxanthellate cnidarians: role of carbonic anhydrase

Carbonic anhydrase (CA, EC 4.2.1.1) activity was detected in 22 species of tropical cnidarians which contain endosymbiotic dinoflagellates (=zooxanthellae). CA activity was 2 to 3 times higher in animal tissue than in algae and ca. 29 times higher in zooxanthellate than azooxanthellate species. It was also higher in the zooxanthellate tentacle tissue than in the azooxanthellate column tissue of the anemone Condylactis gigantea. CA was therefore significantly related to the presence of endosymbiotic algae. Further results indicated that CA functions in the photosynthetic carbon metabolism of zooxanthellate cnidarians as evidenced by (1) low CA activity in shade-adapted and deep water colonies compared to the more productive shallow water, light-adapted colonies of the coral Stylophora pistillata, and (2) the 56 to 85% reduction in photosynthetic carbon assimilation by zooxanthellae in situ in the presence of Diamox, an inhibitor of CA. Although CA has been proposed to function in calcification, its association with zooxanthellae and photosynthetic activity in both calcifying and non-calcifying associations suggests a role in photosynthetic metabolism of algal/cnidarian symbioses. It is proposed that CA acts as a CO₂ supply mechanism by releasing CO₂ from bicarbonate, and enabling zooxanthellae to maintain high rates of photosynthesis in their intracellular environment.